The present invention relates to a method of manufacturing a graded-index plastics optical fiber.
Graded-index plastics optical fibers, suitable for use in a spectrum range covering the visible and the near infrared, are advantageous in that they can be applied to broadband access networks. A graded-index plastics optical fiber can comprise at least one polymer and at least one other compound, with the polymer content being substantially the same throughout the fiber, while the concentration of the other compound varies from the core to the periphery of the fiber in such a manner as to form the desired index gradient. Said compound, which can be referred to as a xe2x80x9cdopantxe2x80x9d or as a xe2x80x9cdiluantxe2x80x9d, comprises at least one substance. Manufacturing such plastics optical fibers is difficult in that it is necessary to obtain a distribution of said compound that varies from the core to the periphery of a plastics optical fiber. The fiber needs to have a refractive index profile of the graded-index type that is as smooth as possible, with total variation in refractive index between the center and the periphery of the fiber generally lying in the range 0.01 to 0.03.
There are two main techniques for making said index gradient.
In a first technique, the index gradient is made at least in part before the optical fiber is spun through a die by diffusing compound(s) in at least one given polymer. Said compound can possibly itself comprise another polymer, at least in part.
In a first implementation of that first technique, such diffusion can take place m said given polymer while it is in the molten state. Thus, patent U.S. Pat. No. 5,593,621 describes manufacturing a plastics optical fiber with an index gradient by forming a cylinder of a first molten polymer and then injecting a transparent material into the central portion of said cylinder, said transparent material being diffusible and non-polymerizahle, and possibly mixed with a second molten polymer that is transparent and of refractive index that is different from that of the first polymer. After said nonpolymerizable material has diffused in the first polymer, the cylinder is spun into an optical fibers. In such all implementation, it is possible to install a plurality of nozzles nested in one another so as to improve the diffusion of said material.
In a second implementation of that first technique such diffusion can take place in said diluted polymer. Thus, patent application JP A-9 243 835 describes the manufacture of a graded-index plastics optical fiber comprising a matrix-forming transparent polymer, a nonpolmerizable second compound having a refractive index higher than that of the transparent polymer, and a nonpolymerizable third compound of refractive index smaller than that of the transparent polymer. The second compound is more concentrated in the core of the fiber. The third compound is more concentrated in the periphery of the fiber. Manufacture consists in preparing layers of various compositions of the second and third compounds, together with a monomer constituting a source for the transparent polymer, said compositions being delivered via a multi-orifice nozzle. After diffusion between the layers and spinning, where diffusion can take place before and during spinning, fiber manufacture is finalized by polymerizing said layers so as to transform the monomer into a polymer.
In the second technique for making a graded-index fiber, the index gradient is made almost entirely after the optical fiber has been spun through a die by applying at least one coating layer. The coating layer can comprise at least one polymer in the molten state. Thus, patent application JP-A-9 133 819 describes manufacturing a graded-index plastics optical fiber continuously by applying molten plastics materials of at least two different types and of decreasing refractive indices around the plastics fiber constituting the higher-index core of the fiber.
Alternatively, the coating layer can also include at least one partially polymerizable compound. Thus, patent application JP-A-9 138 313 discloses a method of manufacturing a graded-index plastics optical fiber by applying a solution obtained by diluting a polymer with a low refractive index compound in a monomer around a plastics core of higher index. The polymerized monomer constitutes the first layer of the polymer of refractive index lower than that of the core. Further layers are deposited in the same manner with refractive indices that decrease progressively from the center towards the periphery. Each layer is polymerized by ultraviolet (UV) treatment after it has been applied. In the example, the viscosity of the solutions applied to the core is adjusted to lie in the range 5000 poises to 10,000 poises, i.e. in the range 500 Pa.s to 1000 Pa.s. To implement the invention, the low refractive index compound is not polymerizable.
Patent application JP-A-6 003 533 describes manufacturing a multilayer graded-index plastics optical fiber continuously by preparing substances having different refractive indices and then applying separate layers, allowing diffusion to take place,between the layers, and then curing said layers.
All of the above manufacturing methods rely on compounds diffusing between adjacent layers, which means that said methods are slow and not very reproducible. This gives rise to major problems in manufacturing graded-index plastics optical fibers on an industrial scale.
The method of manufacture of the invention seeks to manufacture graded-index plastics optical fibers in a manner that is as effective as possible, mainly in terms of speed and reproducibility.
The present invention provides a method of manufacturing a graded-index plastics optical fiber whose refractive index varies continuously between its center and its periphery, said fiber being made from at least one polymer P and at least one compound M1 enabling refractive index to be varied, said method comprising:
preparing two compositions of different refractive indices, the refractive index difference between the two compositions being not less than 5xc3x9710xe2x88x923, each composition comprising at least the polymer P, and a xe2x80x9cfirstxe2x80x9d one of said compositions also having at least the compound M1, a cross-linking photo-initiator being present in at least one of said compositions;
the two compositions are diffused into each other; and
said mixture is spun so as to obtain a graded-index plastics optical fiber;
wherein:
the polymer P and the compound M1 are selected in such a manner that:
said polymer P is of molecular mass lying in the range 1000 to 20,000 and said compound M1 is of molecular mass lying in the range 100 to 1000;
said compound M1 has at least one reactive functional group selected from the group formed by vinyls and acrylates;
the polymer P has at least one reactive functional group selected from the group formed by vinyls and acrylates; and
at least one of the two compounds P and M1 is at least mono-functional, the other of the two compounds P and M1 being at least bi-functional;
wherein
prior to spinning, the two compositions are subjected to active mixing using at least one mixing means so as to obtain continuous variation in the refractive index of the optical fiber; and
wherein
spinning of the graded-index plastics optical fiber is followed by curing giving rise to a cross-linked three-dimensional lattice.
In the invention it is preferred and found more appropriate to use the term xe2x80x9cspinningxe2x80x9d instead of the term xe2x80x9cextrusionxe2x80x9d, even though these two terms are often capable of being confused, particularly when documents are translated. Extrusion applies in general to materials that are solid, whereas spinning applies in general to materials that are not solid, and often liquid. It is not possible to extrude a liquid.
Such active mixing is mixing that takes place with assistance, i.e. does not rely on diffusion only, and the assistance can be given either statically by forcing two compositions to mix using static diffusion means, generally forced flow means, or else dynamically where mixing is performed actively. Such a method has the advantage of being fast, and in particular much faster than when relying on diffusion on its own between the compositions, and it enables a concentration gradient and thus a refractive index gradient to be obtained that is continuous and practically smooth.
The reaction kinetics of cross-linking are generally such that under maximum insolation and complete transformation of the photo-initiator, the gelling time is less than 10 seconds (s), and preferably less than 2 s.
In the method of the invention, spinning of the graded-index plastics optical fiber is followed by polymerization which constitutes cross-linking. Such cross-linking serves advantageously to lock in place at least part of the components of the plastics optical fiber The resulting plastics optical fiber consequently possesses stability over time and temperature. Under such circumstances, generally at least one of the two compositions includes a monomer; furthermore, at least one of the two compositions includes at least one cross-linking initiator, and preferably each of the two compositions includes at least one cross-linking initiator. A cross-linking initiator, e.g. a photo initiator, is a compound which makes it possible to initiate the desired cross-linking reaction, e.g. in response to temperature or to radiation.
Said cross-linking leads to a cross-linked three-dimensional lattice. Such cross-linking advantageously enables practically all of the components of the plastics optical fiber to be locked into place, thereby providing better physical and temperature stability for the resulting plastics optical fiber and for its index gradient.
In an implementation, the second of said compositions includes at least a compound M2 also enabling refractive index to be varied, the compound M2 having a refractive index that is significantly different from the refractive index of M1, having a molecular mass lying in the range 100 to 1000, and including at least one reactive functional group selected from the group formed by vinyls and acrylates.
Preferably, the compounds M1 and M2 are practically identical in viscosity and the mass proportion of said polymer P relative to the ingredients of the composition is practically constant for each of said compositions. This makes the method easier to implement since the varying proportions of the compound(s) M1 and/or M2, serving mainly to modulate the refractive index, has no significant influence on the viscosity of the compositions.
In an embodiment of the method of the invention, the two compositions are mixed at a temperature such that the viscosity of each of the two compositions lies in the range 1 Pa.s to 25 Pa.s, and preferably in the range 5 Pa.s to 15 Pa.s. This serves advantageously to facilitate implementing the method of the invention since such viscosity enables the compositions to be mixed while they are relatively fluid.
In an implementation of the method of the invention, spinning is performed at a temperature such that the viscosity of each of the two compositions is greater than 50 Pa.s, and preferably greater than 100 Pa.s. In like manner, this serves advantageously to facilitate implementing the method of the invention since such viscosity enables compositions to be spun that are relatively stiff.
The reactive functional groups carried by the ingredients M1 and M2 and by the polymer P are selected from the group formed by vinyls and acrylates, i.e. they are selected from the group formed by acrylates, methacrylates, and vinyl ethers, said groups optionally being halogenated at least in part, usually fluorinated and/or chlorinated.
In an implementation of the method of the invention, every component of a said composition is a material that is at least partially halogenated, usually fluorinated and/or chlorinated.
In a variant of the method of the invention, when the compound M2 is present in the second of said compositions, one of the two compounds M1 or M2 is at least partially fluorinated and the other of the two compounds M2 or M1 is at least partially chlorinated or chloro-fluorinated, and thus of refractive index significantly greater than that of the polymer that is at least partially fluorinated.
The invention also provides apparatus for implementing the above-described manufacturing method. In a first embodiment, the mixing means used is a static type mixer. In such a mixer, mixing takes place mainly with the help of a fixed system which modifies the flows of compositions under pressure. It is the shape of the mixer which induces the shape of the gradient. Natural diffusion between the compositions generally serves to smooth the mixture.
In a first embodiment of the mixer, the mixer comprises at least two plates one above the other, each plate being perforated by a plurality of holes, said holes being disposed out of register with one another from one plate to an adjacent plate. In a second embodiment of the mixer, the mixer comprises at least one cartridge of beads. The beads in the cartridge can be practically identical in diameter. It is also possible for at least some of the beads in the cartridge to be of a diameter that is substantially different from at least some other beads in the cartridge. For example, the bead cartridge can have a bead diameter distribution such that bead diameter becomes smaller and smaller going from one end of the cartridge towards the other. In a third embodiment of the mixer, it comprises a plurality of baffles.
In a second embodiment of the apparatus, the mixing means used is a dynamic type mixer. In such a mixer, mixing takes place mainly by the mixing being externally driven, generally mechanically, in addition to natural diffusion between the compositions and the flow under pressure. In an implementation, the mixer comprises at least one shaft having at least one mixer blade is secured thereabout. Said blade can also include at least one means for dispensing at least one of the two compositions. Such a mixer also includes an appropriate fairing to ensure that the presence of the mixer does not give rise to a zone of turbulence within the compositions.
The plastics optical fiber obtained by the method of the invention has the advantage of being usable in a spectral range extending from the visible to the near infrared, while possessing low attenuation, of a few tens of decibels per kilometer (dB/km), over the entire range.
Another advantage of the optical fiber obtained by the method of the invention is that it can be used at temperatures which are high compared with the temperatures at which plastics optical fibers have been used in the prior art, and it can be used up to at least 125xc2x0 C., with this being made possible by its cross-linked nature.
The fiber obtained in this way possesses a refractive index profile of the practically smooth graded-index type in which refractive index variation between the center and the periphery of the fiber generally lies in the range 0.01 to 0.03. The diameter of the fiber obtained in this way generally lies in the range 100 xcexcm to 1 mm.